Various fields of industry including those engaged. in the manufacture of electric appliances for domestic use and machines and furniture for office use have recently come to use to a greater extent a weldable black steel sheet having an excellent appearance which can be obtained by the continuous and rapid treatment of a steel sheet plated with zinc or an alloy thereof. A great deal of research and development work has, therefore, been made to obtain such black steel sheets, and includes efforts made to obtain an effective method for the blackening treatment of a steel sheet, and an effective solution which can be used for such treatment.
The following is a summary of the methods which have hitherto been proposed for forming a black film on a steel sheet:
(a) A solution of a resin containing a black pigment, such as carbon black, is applied onto a steel sheet by e.g. spraying or roll coating to form thereon a film having a thickness of several tens of microns. PA1 (b) A film existing on a steel sheet as a result of plating is caused to undergo reaction or electrolysis to develop a black film. This method includes a number of modes as will hereunder be set forth: PA1 (c) A treating solution obtained by adding an organic dye to an aqueous solution of potassium sodium silicate is applied onto a surface plated with zinc or an alloy thereof to form a corrosion-resistant and closely adhering film thereon (Japanese Patent Publication No. 30593/1980). PA1 (a) This is a common method of coating used to form on a steel sheet a surface coating, consisting of a single layer and having usually a thickness of at least 10 microns. This thickness is too large to allow the welding of the steel sheet. The thickness of the film which allows the welding of the steel therethrough is from 0.2 to 3 microns. It is difficult to form a satisfactorily black film having a thickness not exceeding three microns, even if the solution may contain the black pigment, such as carbon black, at the maximum possible concentration. In other words, the use of any such blackening agent has been found unable to form any film that is satisfactorily black and yet keeps weldable the steel sheet on which the film has been formed. PA1 (b) (1) The solution containing Ag ions is expensive. Moreover, this method spends as long a time as several tens of seconds in forming a black film and is, therefore, inapplicable to the continuous treatment of a strip which need be finished as rapidly as within five seconds. PA1 (2) The black film made by this method is unsatisfactory in workability. Insofar as carbon black is a conductive pigment, the film is electrically conductive and is, therefore, low in corrosion resistance. Moreover, the film is unsatisfactory in blackness. PA1 (3) and (4) These methods are both uneconomical, since they rely upon the dissolution of metal from the plated layer on the steel sheet. Moreover, the metal ions as dissolved from the plated layer deteriorate the solution for the blackening treatment and can present a serious problem to the continuous treatment. Furthermore, both of the methods have only a limited scope of application, i.e. (3) is applicable only to a steel sheet plated with a Zn--Co, Zn--Ni or Zn--Mo alloy, while (4) is applicable only to a sheet plated with a Zn--Ni alloy. PA1 (5) The black film made by this method is unsatisfactory in formability. PA1 (6) The black film which is formed by this method is unsatisfactory in adhesive strength, and is also low in corrosion resistance, insofar as a metal having a relatively noble or high potential is deposited on a metal having a relatively base or low potential (i.e. on zinc or an alloy thereof). PA1 (c) this method is not intended for forming a film having an excellently black surface, nor is it intended for imparting weldability, insofar as the disclosure does not contain any specific reference to the film thickness. Moreover, as the film is basically composed of potassium sodium silicate, the hardened film is unsatisfactory in lubricating property as required during press forming, and is, therefore, unsuitable for any steel sheet that is used for making electrical appliances for domestic use, office machines or furniture, etc. Moreover, the disclosure does not specifically define the organic dye to be used. There are a very large number of organic dyes which are widely different from one another in various properties including quality of its color being unchangeable after a long exposure to light (light fastness), tinting strength and solubility in a solvent. Some dyes are superior to others in e.g. tinting strength, but inferior in light fastness. It is, therefore, needless to say that all of the organic dyes are not equally useful. PA1 (1) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin and 1 to 200 parts by weight of a water-soluble black dye as a blackening agent, the black film having a thickness of 0.3 to 3.0 microns. PA1 (2) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble black dye as a blackening agent and 1 to 100 parts by weight of a solid lubricant, and the black film having a thickness of 0.3 to 3.0 microns. PA1 (3) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 100 parts by weight of a water-soluble black dye as a blackening agent and 1 to 100 parts by weight of a granular rust-inhibitive pigment, and the black film having a thickness of 0.3 to 3.0 microns. PA1 (4) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 200 parts by weight of a water-soluble black dye as a blackening agent, 1 to 100 parts by weight of a solid lubricant and 1 to 100 parts by weight of a granular rust-inhibitive pigment, the black film having a thickness of 0.3 to 3.0 microns. PA1 (5) A weldable black steel sheet as set forth at (1), (2), (3) or (4) above, wherein the water-soluble black dye is at least one dye selected from the group consisting of azo, azomethine, quinoline, ketoneimine, fluorone, nitro, xanthene, acenaphthene, qionophthalone, anthraquinone, aminoketone, methine, perylene, coumarin, perionone, triphenyl, triallylmethane, phthalocyanine, isochlorophenol and azine dyes. PA1 (6) A weldable black steel sheet as set forth at (5) above, wherein the black dye is a mixture of a water-soluble black azo dye composed of a complex metal salt, and another water-soluble black dye. PA1 (7) A weldable black steel sheet as set forth at (1), (2), (3) or (4) above, wherein the water-soluble black dye is a black azo dye composed of a complex metal salt. PA1 (8) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, and `x` parts by weight of a water-soluble black azo dye composed of a complex metal salt as a blackening agent, the black film having a thickness of `y` microns, the `x` being a value satisfying formulas 1 and 2, while the `y` is a value satisfying formulas 1 and 3: EQU (x-1)(Y-0.2).gtoreq.7 (1) EQU 1&lt;x.ltoreq.200 (2) EQU 0.2&lt;y.ltoreq.3 (3) PA1 (9) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 100 parts by weight of a solid lubricant and `x` parts by weight of a water-soluble black azo dye composed of a complex metal salt as a blackening agent, the black film having a thickness of `y` microns, the `x` being a value satisfying formulas 1 and 2, while the `y` is a value satisfying formulas 1 and 3: EQU (x-1)(Y-0.2).gtoreq.7 (1) EQU 1&lt;x.ltoreq.200 (2) EQU 0.2&lt;y.ltoreq.3 (3) PA1 (10) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 100 parts by weight of a granular rust-inhibitive pigment and `x` parts by weight of a water-soluble black azo dye composed of a complex metal salt as a blackening agent, the black film having a thickness of `y` microns, the `x` being a value satisfying formulas 1 and 2, while the `y` is a value satisfying formulas 1 and 3: EQU (x-1)(Y-0.2).gtoreq.7 (1) EQU 1&lt;x.ltoreq.200 (2) EQU 0.2&lt;y.ltoreq.3 (3) PA1 (11) A weldable black steel sheet having a surface plated with zinc or a zinc alloy, and carrying a chromate film formed on the plated surface and having a coating weight of 10 to 200 mg/m.sup.2 in terms of metallic chromium, and a black azo film formed on the chromate film from a composition comprising 100 parts by weight of a water-soluble or -dispersible thermosetting resin as a base resin, 1 to 100 parts by weight of a solid lubricant, 1 to 100 parts by weight of a granular rust-inhibitive pigment and `X` parts by weight of a water-soluble black azo dye composed of a complex metal salt as a blackening agent, the black film having a thickness of `y` microns, the `x` being a value satisfying formulas 1 and 2, while the `y` is a value satisfying formulas 1 and 3: EQU (x-1)(Y-0.2).gtoreq.7 (1) EQU 1&lt;x.ltoreq.200 (2) EQU 0.2&lt;y.ltoreq.3 (3) PA1 (12) A weldable black steel sheet as set forth at (8) to (11), wherein the black film is formed from a composition comprising `x` parts by weight of a water-soluble black azo dye composed of a complex metal salt as a blackening agent against 100 parts by weight of a base resin, and have a thickness of `y` microns, said `x` being a value satisfying formulas 1' and 2', while said `y` is a value satisfying formulas 1' and 3': EQU (x-5)(y-0.65).gtoreq.6 (1') EQU 5&lt;x.ltoreq.120 (2') EQU 0.65&lt;y.ltoreq.2.5 (3') PA1 (13) A weldable black steel sheet as set forth at (2), (4), (5), (6), (7), (9), (11) or (12), wherein the solid lubricant is at least one material selected from the group consisting of hydrocarbon compounds such as polyolefin wax, fluororesins, fatty acid amides, metallic soaps, metal sulfides such as molybdenum disulfide, graphite, graphite fluoride, boron nitride and polyalkylene glycols. PA1 (14) A weldable black steel sheet as set forth at (3), (4), (5), (6), (7), (10), (11), (12) or (13), wherein the granular rust-inhibitive pigment is at least one material selected from the group consisting of sparingly soluble chromium compounds and silica.
(1) A chromating solution containing Ag ions is used to form a black chromate film (as proposed in Japanese Patent Application Laid-Open No. 193376/1983); PA2 (2) A black resin film composed mainly of carbon black is electrolyzed (Japanese Patent Application Laid-Open No. 62996/1981); PA2 (3) A steel sheet which has been electroplated with a Zn--Co, Zn--Ni or Zn--Mo alloy is subjected to anodic treatment (Japanese Patent Publication No. 38276/1986); PA2 (4) A steel sheet which has bee plated with a Zn--Ni alloy is subjected to dipping, spraying or anodic treatment with a solution containing nitric acid or a nitrate group to develop a black surface (Japanese Patent Publication No. 30262/1987); PA2 (5) A black plated film is formed by cathodic treatment (e.g. Japanese Patent Application Laid-Open No. 263995/1987); and PA2 (6) Immersion plating is carried out on a surface plated with zinc or an alloy thereof to deposit thereon a metal having a nobler potential (e.g. Japanese Patent Application Laid-Open No. 89879/1987)
All of these methods do, however, have their own drawbacks, as will hereunder be pointed out.
Under these circumstances, it is an object of this invention to provide a black steel sheet coated with a film which allows the welding of the sheet therethrough and yet exhibits an excellent black surface. The weldable black steel sheet of this invention is an improvement over the product of the known method as described at (a) above in which a black pigment, such as carbon black, is employed to form a black film. The steel sheet of this invention is, therefore, free from any of the problems encountered by the known methods relying upon reaction or electrolysis as described at (b) above, including the deterioration of the treating solution by the dissolved metal ions, the loss of the plating material, and the scope of application limited by the underlying plating material. Moreover, the steel sheet of this invention can be produced continuously and rapidly by using an existing continuous strip coating and baking line.
It is another object of this invention to provide a black steel sheet which is not only weldable and excellent in surface blackness, but also excellent in corrosion resistance, adhesive strength and formability.
Although the organic solvents have usually been used in a wide variety of coating compositions, the harmfulness of their vapor to the human body has turned out to be a big environmental problem. The latest tendency is, therefore, toward the use of an aqueous coating composition. Under these circumstances, it is still another object of this invention to provide a black steel sheet coated with a black film formed from an aqueous composition.